Usually, a vehicle bearing apparatus is adapted to freely rotatably support a wheel hub to mount the wheel via a rolling bearing. An inner ring rotation type is adopted for a driving wheel and both inner ring rotation and outer ring rotation types are adopted for a driven wheel. A double row angular contact ball bearing is widely used in such a bearing apparatus. Reasons for this is that it has a desirable bearing rigidity, high durability against misalignment and small rotation torque required for fuel consumption. The double row angular contact ball bearing has a plurality of balls interposed between a stationary ring and a rotational ring. The balls are contacted by the stationary and rotational rings at a predetermined contact angle.
The vehicle wheel bearing apparatus is broadly classified into a first through fourth generation structure. A first generation structure includes a wheel bearing with a double row angular contact ball bearing fit between a knuckle forming part of a suspension and a wheel hub. A second generation structure includes a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member. A third generation structure includes one of the inner raceway surfaces directly formed on the outer circumference of the wheel hub. A fourth generation structure includes the inner raceway surfaces directly formed on the outer circumferences of the wheel hub and a constant velocity universal joint. In the description below, the term “outer side” (left hand side in the drawings) of the apparatus denotes a side that is positioned outside of the vehicle body. The term “inner side” (right hand side in the drawings) of the apparatus denotes a side that is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.
In the prior art wheel bearing apparatus formed with a double row rolling bearing, the bearing arrangements in both the left and right rows are the same. Thus, it has sufficient rigidity during straight way running. However, optimum rigidity cannot always be obtained during curved way running. Accordingly, the positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts on substantially the middle between the rows of bearing balls during straight way running. However, larger radial loads and larger axial loads are applied onto the vehicle axles, on the side opposite, during running in a curved direction (i.e. axles of the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity of the bearing row on the outer side than the inner side of the bearing row in order to improve the durability and strength of the bearing apparatus. Thus, the vehicle wheel bearing apparatus shown in FIG. 11 is known to have a high rigidity without enlargement of the bearing apparatus.
The vehicle wheel bearing apparatus 50 is formed by a double row angular contact ball bearing including an outer member 51 integrally formed, on its outer circumference, with a body mounting flange 51c to be mounted on a knuckle (not shown) of a vehicle. Its inner circumference includes double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with a wheel mounting flange 53 integrally formed at one end to mount a wheel (not shown). One inner raceway surface 52a is formed on its outer circumference opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. Its outer circumference has the other inner raceway surface 54a opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row balls 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the balls 56, 57.
The inner ring 54 is axially immovably secured by a caulked portion 52c. The caulked portion 52c is formed by plastically deforming the cylindrical portion 52b of the wheel hub 52 radially outward. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55. The seals 60, 61 prevent the leakage of grease contained within the bearing apparatus and the entering of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer side ball group 56 is set larger than a pitch circle diameter D2 of the inner side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than that of the inner raceway surface 54a of the inner ring 54. The outer raceway surface 51a of the outer side of the outer member 51 is larger than that of the outer raceway surface 51b of the inner side of the outer member 51. Also, the number of outer side balls 56 is larger than that of the inner side balls 57. By setting the pitch circle diameter D1 of the outer side larger than the pitch circle diameter D2 of the inner side (D1>D2), it is possible to obtain a large rigidity of the bearing apparatus 50 that extends its life. Patent Document 1: Japanese Laid-open Patent Publication No. 108449/2004
In the prior art bearing apparatus 50, the pitch circle diameter D1 of the outer side ball group 56 is set larger than the pitch circle diameter D2 of the inner side ball group 57. Thus, the number of the balls of outer side is larger than the number of balls of inner side. This is to increase the bearing rigidity. However, a problem exists on how to effectively increase the bearing rigidity while suppressing an increase of the weight of the bearing apparatus.
Also in the prior art bearing apparatus 50, a root portion 62 of the of the body mounting flange 51c of the outer member 51 interferes with an internal thread portion 51d of the body mounting flange 51c when the outer side wall thickness of the outer member 51 is increased in order to increase its rigidity corresponding to the enlargement of the pitch circle diameter D1 of the outer side ball group 56. This makes tapping of the internal thread portion 51d difficult and thus the increase of rigidity of the outer member 51 is limited.